As a glass sheet having a higher heat insulating performance than a pair of glass sheets, there is known a double glazing comprising a pair of glass sheets combined together with an air layer acting as a heat insulating layer being interposed therebetween. However, this type of glass panel suffers the problem of its own significant thickness which tends to deteriorate the aesthetic appearance including that of a sash. Then, in order to achieve a higher heat insulating performance with a smaller thickness, there has been proposed a glass panel in which a plurality of spacing members are interposed between a pair of glass sheets and a heat-fusible outer periphery sealing portion (e.g. low-melting glass) is provided along the entire outer periphery of the two glass sheets for sealing the space between the glass sheets under a pressure-reduced condition, so that the glass panel may be formed thinner, yet have a lower heat transmission coefficient.
And, for forming the outer periphery sealing portion, paste of low-melting glass is disposed at the outer peripheral edges of the two glass sheets and then heated above the fusing point of the low-melting glass, so that fused low-melting glass is caused to extend over between the outer peripheral edges of the two glass sheets. Then, the temperature is returned to the normal temperature to solidify the low-melting glass, so that this solidified low-melting glass forms the outer periphery sealing portion.
According to a conventional glass panel of the above-noted type, it has been proposed to employ a same kind of glass sheets (e.g. float glass) as the pair of glass sheets or to employ a wire glass as one of the glass sheets in case the panel is to be used in a fire retarding area.
However, the above-described conventional glass panel suffers the following problem.
Namely, the formation of the outer periphery sealing portion requires elevating of the atmosphere temperature of the glass panel and then returning the temperature to the normal temperature, as described above. In the course of this, each of the pair of glass sheets is expanded and contracted, depending on the atmosphere temperature. Such expansion and contraction of glass sheet is affected by its coefficient of linear expansion. For instance, if the coefficients of linear expansion of the pair of glass sheets are different from each other, when the atmosphere temperature is elevated, the amount of expansion will be greater in the glass sheet having the greater linear expansion coefficient.
On the other hand, when the atmosphere temperature is returned to the normal temperature, both of the glass sheets will be contracted respectively to their original dimensions.
When the outer peripheral edges of the pair of glass sheets are joined together at the outer periphery sealing portion, this operation is effected under the atmosphere of the elevated temperature. Hence, the two glass sheets will be combined together while they keep the different amounts of expansion therein. Then, as the atmosphere temperature gradually returns to the normal temperature, the glass sheet having the higher linear expansion coefficient will be contracted more than the other glass sheet.
As a result, the difference between the contraction amounts of the two glass sheets will appear as a flexion, whereby the glass panel may be warped. If the amount of flexion is significant, the glass panel may be broken due to the atmospheric pressure when the inside thereof is pressure-reduced.
With the conventional glass panel described above, if different kinds of glass sheets are employed as the pair of glass sheets, the two glass sheets may be warped and broken. Or, even when they are not broken, a significant internal stress will remain within the two glass sheets, so that a required strength cannot be obtained.
An object of the present invention is to solve the above problem by providing a glass panel comprised of glass sheets having different expansion coefficients, with which panel an appropriate strength may be readily obtained and which functions stably for a long period of time in spite of the pressure-reduced inside thereof.